Manchester United playmaker Shinji Kagawa is reportedly undergoing scans to determine the extent of the back injury suffered while on international duty with Japan.
Kagawa was schedule to start Tuesday's 1-0 Group B win over Iraq but withdrew just an hour before kick-off and was left on the substitutes bench by coach Alberto Zaccheroni.
Speaking after the game, the 23-year-old had insisted the injury wouldn't keep him out of action for too long, but according to Goal.com, Kagawa was still feeling discomfort in his back when he returned to United's Carrington training ground and was sent for scans on Thursday.
The midfielder is believed to be a major doubt for Saturday's Premier League game at home to Wigan.
A Japan Football Association official had on Tuesday told Japan's Daily Sports: "He felt uncomfortable in his lower back in practice yesterday, and we decided to wait a night and see how it looked, but he's not at 100% so we're leaving him off.
"He didn't go to the hospital."
Kagawa told Nikkan Sports before flying back to England: "It [the injury] happened in yesterday's practice. I was holding the ball and went to make a move and in that instant it started to hurt.
"I won't take long [to recover]."
United boss Sir Alex Ferguson could also be without striker Robin van Persie, who is struggling with a thigh problem picked up while away with the Netherlands squad this week.